1. Field of the Invention
In the anionic (alkaline-catalyzed) polymerization of 2-pyrrolidone, alkali metal hydroxides, particularly sodium and potassium hydroxide, have commonly been used as sources of alkaline catalyst. The reaction of the hydroxide with 2-pyrrolidone produces the alkali metal pyrrolidonate catalyst, and, as a by-product, water. The reaction is believed to be represented by, e.g., for KOH, the following equation: ##STR1##
The reaction is typically carried out by contacting the hydroxide in solid form, or in aqueous solution, with excess liquid 2-pyrrolidone to form a solution of alkali metal pyrrolidonate and water in 2-pyrrolidone. The solution is then dehydrated to produce a substantially anhydrous solution of alkali metal pyrrolidonate catalyst in 2-pyrrolidone. Elevated temperatures are used to effect dissolution of the hydroxide in 2-pyrrolidone and subsequent dehydration of the solution. The catalytic properties of the solution are normally found to be sensitive to temperature and water content.
Evaporative and distillative dehydration conditions lend themselves to the loss of appreciable amounts of 2-pyrrolidone with the water removed from the catalyst solution, as well as causing hydrolysis of 2-pyrrolidone remaining in the catalyst solution. Both the loss of 2-pyrrolidone and its hydrolysis are the principal causes of inefficiency in catalyst preparation by this method.
Hydrolysis of the sensitive 2-pyrrolidone ring, which occurs even under mild conditions of heating aqueous 2-pyrrolidone, produces the polymerization inhibitor 4-aminobutyric acid (also see U.S. Pat. No. 3,778,402). On the other hand, dehydration of 2-pyrrolidone to dimer, which is said to occur even under mild conditions of prolonged heating of anhydrous 2-pyrrolidone, produces polymerization initiators which cause "runaway polymerization" with the production of low-molecular-weight polypyrrolidone (see U.S. Pat. No. 3,184,476).
2. Prior Art
It has been the usual practice to dissolve solid hydroxide, typically in the form of KOH pellets (85% KOH), in excess 2-pyrrolidone and to remove water under conditions of reduced pressure (see U.S. Pat. Nos. 2,739,959 and 3,721,652). U.S. Pat. No. 2,739,959, for example, discloses the production of catalyst for the anionic polymerization of 2-pyrrolidone by contacting 1 part of KOH pellets with excess (200 parts) of 2-pyrrolidone and removing the water of reaction by distilling off 20 parts of the mixture at a temperature between 90.degree. and 120.degree. C. and a pressure between 0.5 and 10 mm. Potassium hydroxide may also be contacted with excess 2-pyrrolidone in the form of an aqueous solution (see U.S. Pat. No. 3,778,402).
U.S. Pat. No. 3,563,980 discloses a process for preparing a mixture of lactam and an alkali metal compound of this lactam by reacting the hydroxide with the lactam and evaporating the water of reaction by heating in vacuo. It is noted therein that the drawback to the use of a hydroxide is the formation of water, because it must be removed quickly by heating in vacuo to avoid the occurrence of side reactions whose products weaken the activity of the catalyst. It is also noted therein that the rapid removal of water results in the loss of considerable amounts of lactam as well as water.
In the process of U.S. Pat. No. 3,681,293, the catalyst solution is passed to a wiped-film evaporator for dehydration. The evaporator is operated at reduced pressure at as low a temperature as possible. In order to assure that the solution is dehydrated, a mixture of water and 2-pyrrolidone is removed. The problem of hydrolysis of 2-pyrrolidone by KOH and water at elevated temperatures is dealt with by using two evaporators in series, with only the last evaporator operated at temperatures over 80.degree. C.